Improving Salinity Tolerance of Plants Through Conventional Breeding and Genetic Engineering: an Analytical Comparison
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Biotechnology Advances 27 (2009) 744–752 Contents lists available at ScienceDirect Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv Research review paper Improving salinity tolerance of plants through conventional breeding and genetic engineering: An analytical comparison Muhammad Ashraf ⁎, Nudrat Aisha Akram Department of Botany, University of Agriculture, Faiasalabad 38040, Pakistan article info abstract Article history: The last century has witnessed a substantial improvement in yield potential, quality and disease resistance in Received 18 February 2009 crops. This was indeed the outcome of conventional breeding, which was achieved with little or no Received in revised form 26 May 2009 knowledge of underlying physiological and biochemical phenomena related to a trait. Also the resources Accepted 26 May 2009 utilized on programs involving conventional breeding were not of great magnitude. Plant breeders have also Available online 10 June 2009 been successful during the last century in producing a few salt-tolerant cultivars/lines of some potential crops through conventional breeding, but this again has utilized modest resources. However, this approach Keywords: fi Genetic engineering seems now inef cient due to a number of reasons, and alternatively, genetic engineering for improving crop Salt tolerance salt tolerance is being actively followed these days by the plant scientists, world-over. A large number of Conventional breeding transgenic lines with enhanced salt tolerance of different crops can be deciphered from the literature but up Ion homeostasis to now only a very few field-tested cultivars/lines are known despite the fact that considerable resources Osmoprotectants have been expended on the sophisticated protocols employed for generating such transgenics. This review Antioxidants analytically compares the achievements made so far in terms of producing salt-tolerant lines/cultivars through conventional breeding or genetic engineering. © 2009 Elsevier Inc. All rights reserved. Contents 1. Introduction .............................................................. 744 2. Selection and breeding for salinity tolerance — A conventional approach .................................. 745 3. Engineering plants for enhanced salt tolerance: transgenic approach.................................... 745 4. Transgenic plants for ion transporters .................................................. 745 5. Transgenic plants for compatible organic solutes ............................................. 747 6. Transgenic plants for enhanced antioxidant production .......................................... 747 7. Challenges and prospects ........................................................ 750 References ................................................................. 751 1. Introduction “biotic approach” (Epstein, 1977; Kingsbury and Epstein, 1984; Ashraf, 1994). The term “biosaline agriculture” is also analogous to this. The Although salinity is a major problem for agriculture in arid and biotic approach was considered as the most efficient and economical semi-arid regions of the world, this menace can be observed in other means of utilizing salt-affected soils by many scientists (Epstein, 1977; areas as well. This environmental adversary has caused considerable Shannon, 1985; Ashraf, 1994) and hence a permanent solution of the crop yield losses and hence very low crop productivity. However, to problem. combat this peril, long ago, two key strategies for the utilization of Producing crops tolerant to salinity stress is imperative to run into salt-affected lands, i.e., reclamation of salt-affected soils and growing the growing food demand through sustainable agriculture. This could be halophytes or salt-tolerant crops/cultivars on such soils, were achieved either through conventional selection and breeding or through proposed (Epstein, 1977). The latter approach was referred to as the modern molecular biology approaches. Plant breeders have succeeded to some extent in producing salt-tolerant lines/cultivars of some crops through conventional breeding, but the main reason of the limited ⁎ Corresponding author. Tel.: +92 419200312. success through this strategy has been due to the fact that the magnitude E-mail address: [email protected] (M. Ashraf). of genetic variation in the gene pools of most important crops is very 0734-9750/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2009.05.026 M. Ashraf, N.A. Akram / Biotechnology Advances 27 (2009) 744–752 745 low. However, alternatively, transgenic approach for improving crop salt are special in a way that they are developed from only a single plant tolerance is being actively pursued these days by the plant scientists, cell. Transgenic plants have been developed through different genetic world-over, and a number of transgenic lines of different crops can be engineering techniques to contain desirable traits, including resis- easily deciphered from the literature. tance to pests, pesticides, diseases or adverse environmental condi- This review analytically compares the achievements made so far in tions, improved nutritional value, and enhanced product shelflife. terms of producing salt-tolerant lines/cultivars through conventional Despite a number of social, political and legal concerns, many breeding or genetic engineering. It also highlights some of the countries are now allowing transgenic crop production in conjunction challenges confronting genetic engineers because one can easily with their conventional crop production. In view of a report by The assess that only few lines/cultivars of different crops generated International Service for the Acquisition of Agro-Biotech Applications through genetic engineering and molecular biology approaches have (ISAAA) the top five countries growing transgenic crops in 2005, were thrived well under natural field conditions, though most of the claims USA, Argentina, Brazil, Canada, and China (ISAAA, 2006). However, of the scientists regarding the good performance of salt-tolerant transgenic approach is being effectively pursued by plant scientists transgenic lines are based on growth room and glasshouse trials. these days not only to improve quality and yield but also to increase tolerance to biotic and abiotic stresses in a number of crops. Since the 2. Selection and breeding for salinity tolerance — A salt tolerance trait involves a number of minor genes (quantitative conventional approach trait loci) so it is very complex. Thus, improving crop salt tolerance by genetic engineering is not so easy. Nonetheless, for improving salt Although considerable progress was made during the 20th century tolerance trait in various crops through genetic engineering plant to improve crop yield and quality through conventional breeding, we biologists have focused much on genes that encode: ion transport can see relatively little work on improving the resistance of crops proteins, compatible organic solutes, antioxidants, heat-shock and against abiotic stresses, especially salinity stress. Perhaps this has been late embryogenesis abundant proteins, and transcription factors for due to the main reason that plant breeders focused more on improving gene regulation (Ashraf et al., 2008). However, in the present review crop yield and quality than on improving stress tolerance. Nonetheless, we have discussed only the transgenic lines produced for the plant breeders had been carrying out various breeding programs overexpression of ion transport proteins, compatible organic solutes, during the last century wherein they had wisely exploited the genetic and antioxidants. variation of crops at intra-specific, inter-specific, and inter-generic levels so as to produce salt-tolerant lines/cultivars. Resultantly, they 4. Transgenic plants for ion transporters were able to release a few salt-tolerant lines/cultivars of different potential crops through conventional breeding. It is imperative to note High levels of salt in the growth medium of plants affect their that most of the lines/cultivars produced through conventional growth and development and ultimately, the yield. To prevent salt breeding were tested under natural field conditions. For example, damage, plants have developed different mechanisms to reduce Na+ some lines/cultivars of alfalfa (Medicago sativa L.) such as AZ-Germ Salt uptake or compartmentalize Na+ into vacuoles. In view of a number of II (Dobrenz et al., 1989), AZ-90NDC-ST (Johnson et al., 1991), AZ- reports it is evident that Na+ moves passively through a general cation 97MEC and AZ-97MEC-ST (Al-Doss and Smith, 1998), ZS-9491 and ZS- channel from the saline growth medium into the cytoplasm of plant 9592 (Dobrenz, 1999) were tested under natural field conditions. cells (Blumwald, 2000; Jacoby,1999; Mansour et al., 2003; Ashraf et al., Similarly, two salt-tolerant lines/cultivars of bread wheat (Triticum 2008), but there is also a sound evidence that Na+ can move actively aestivum L.) such as S24 (Ashraf and O'Leary, 1996) and KRL1-4 through Na+/H+ antiporters, which exchange Na+ or Li+ for H+ (Hollington, 2000) were evaluated on natural salt-affected soils. (Ratner and Jacoby, 1976; Niu et al., 1993; Shi et al., 2003), and the These few examples depict that there has been a limited success in regulation of Na+ uptake and accumulation into plant cells is relied on producing salt-tolerant cultivars of different potential crops using the the regulation of proton pumps and antiporters operating